State of the Bay Report 2011-Final.pdf - Anchor Environmental

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State of Saldanha Bay & Langebaan Lagoon 2011 Fish 9.3.4 Multivariate analysis of spatial and temporal trends in fish communities The use of multivariate statistical techniques allows for the analysis of any patterns in the complete fish community, taking account of both the community species composition, and the abundance of each species. In the 2009 State of the Bay report, multivariate analyses showed that on average, the fish communities from each of the three areas (Small Bay, Big Bay and Langebaan Lagoon) are significantly different from each other. This was related to environmental differences between the three areas. It was concluded that although the whole Saldanha Bay- Langebaan Lagoon system is connected, the near-shore environment in one area (i.e. Small Bay, Big Bay or the Lagoon) on average, appears more suitable to the juveniles of particular species than the other areas. The statistically significant differences in the fish communities found in the three main areas (Small Bay, Big Bay and Langebaan Lagoon), as well as the similarities between sites within each of these areas, supported the analysis of temporal trends (which provide information on changes in the health of the marine environment) on an area specific basis. The 2010 State of the Bay report also reported on the separation of the different sites within each area, based on dissimilarities in the fish community between sites. This separation is similar to the overall trend in fish communities throughout the bay and lagoon, a pattern relating to the degree of exposure of each site was evident, from the most exposed sites through to the most sheltered samples. In this report, analysis focussed on detecting any differences between years, taking account of the established inter-site variability by using a two factor (sites and years as factors) PERMANOVA design. The MDS plot for Small Bay shows that samples from most years grouped centrally, suggesting little change in the fish community over time (Figure 9.8). Fish samples taken at some sites during 2005 and 2011 are however outliers, indicating that these are dissimilar to the majority of other years. Transform: Fourth root Resemblance: S17 Bray Curtis similarity 2D Stress: 0.18 year 94 2005 2007 2008 2009 2010 2011 Figure 9.8. Multidimensional scaling plots showing similarities between the fish communities sampled at four sites within Small Bay during 1994, 2005, 2007, 2008, 2009, 2010 and 2011 sampling events. 208 ANCHOR e n v i r o n m en t a l
State of Saldanha Bay & Langebaan Lagoon 2011 Fish A two way PERMANOVA indicated significant differences in the fish community between sample years (Pseudo F = 2.7, P< 0.0001) and between sites (Pseudo F = 9.2, P< 0.0001) and a significant interaction effect (Pseudo F = 2.9, P< 0.0001). Pairwise tests indicate that there are significant differences between all years sampled and at least two of the other annual sampling events (Table 9.4). This is indicative of the high natural variability in the surfzone fish community inhabiting Small Bay. We suspect, given the orientation of Small Bay facing into the prevailing southerly wind, that short term meteorological changes, (such as a wind change to a northerly or westerly) that are common during April and strongly influence the nature of the surfzone, are the primary drivers of this observed variability. The only way to account for this natural variability is to sample more intensively, by replicating the survey within years, sampling the same sites on different days within each survey and repeating it over several weeks. This is unfortunately not logistically possible, but trends in the ecological health of the Bay (for fish at least) can still be inferred from any long term consistent trends over time. It is clear from the MDS plot that only some of the 2005 and 2011 samples collected in Small Bay were the most dissimilar from the other annual samples and there is no consistent trend that may be indicative of increasing or decreasing ecosystem health (Figure 9.8). It must be noted that as with the 2005 samples, there is also high inter-sample variability (spread of data in the MDS plot) within the 2011 samples. (Some samples group with those from other survey periods, some are outliers). The 2011 sites that are outliers include the small craft harbour (typically different from the other Small Bay sites) and the Campsite sites (where very high catches of harders were made during 2011 sampling), and this variability does not necessarily represent declining ecosystem health at these sites. Although the 2011 Small Bay samples were not significantly different from the 2010 samples, the fish community overall was significantly different from that sampled during 1994, 2007 and 2009. SIMPER analyses identified higher abundance of harders, and decreased average abundance of silversides, white stumpnose, Cape sole, blacktail and gobies in the 2011 samples as the dominant causes (>80%) of dissimilarity between the 2011 samples and the significantly different 1994, 2007 and 2009 samples. Although none of these species had disappeared from catches in Small Bay during 2011, they were on average substantially less abundant than in nearly all of the earlier surveys and this is somewhat concerning. Table 9.4. Results of the multivariate PERMANOVA pairwise tests between Small Bay fish samples collected in different years. NS: not significant, *: P < 0/05, **: P < 0.01 1994 2005 2007 2008 2009 2010 1994 2005 * 2007 ** NS 2008 NS * NS 2009 ** NS NS * 2010 NS * NS * * 2011 * NS * NS * NS Within Big Bay too, little grouping of sampling years in the MDS plot is evident with the 2008 and 2005 outliers representing a few of Plankiesbaai and North Bay samples (Figure 9.9). All of the 2011 samples are distributed well within the range of samples collected in earlier years, indicating no substantial changes in the Big Bay fish communities overall at sampled sites. The mixed model PERMANOVA test did however, indicate significant differences between sites (Pseudo F = 7.6, P< 0.001), between sampling events (Pseudo F = 14.9, P< 0.05) and a significant interaction effect (Pseudo F = 4.1, P< 0.001). Pairwise testing showed that only the Big Bay fish samples collected during 1994 were significantly different from those collected during 2007 and 2009. Big Bay fish 209 ANCHOR e n v i r o n m en t a l
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